An optical disk of the ZCLV (Zone Constant Linear Velocity) format is radially divided into a plurality of zones, and recording and regeneration in each zone are carried out by the CAV (Constant Angular Velocity) system. The ZCLV system has a feature that a frequency of recording and regeneration signal is constant, and information can be densely recorded in an outer periphery in the same manner as in an inner periphery. A DVD-RAM adopts this system, and a high density in the first generation at 2.6 GB/surface and in the second generation at 4.7 GB/surface, has been realized. In this system, since the linear velocities in the inner and outer peripheries are substantially constant, when an optical head position is in the inner periphery, a disk rotation velocity is increased, and reversely when in the outer periphery, the disk rotation velocity is reduced. Accordingly, in the case of a seek, not only a movement of the optical head but also the disk rotation velocity must be controlled. For example, in the case of the optical disk having diameter of 120 mm, the disk rotation velocity when the seek is performed from radius 25 mm to radius 50 mm is ½, much time was required to control a spindle motor, and a rotation velocity control time occasionally determined a seek time. Furthermore, there occurs a problem that power consumption increases for controlling the rotation velocity of the spindle motor. In Japanese Patent Laid-Open No. 11-296858 gazette, a method is proposed for adjusting a recording power in conformity to an actual linear velocity even in the case where an actual rotation velocity cannot sufficiently follow a target rotation velocity immediately after the seek, or the like, and the linear velocity deviates from the target velocity.
If the rotation velocity is made to be substantially constant in the inner and outer peripheries, it is possible to shorten the rotation velocity control time at the time of the seek, and also to decrease power consumption required for controlling the rotation velocity of the spindle motor. Then, in order to realize a high-transfer rate when regenerating data, clock frequency in the inner periphery in which the linear velocity becomes slow is set lower, and the clock frequency in the outer periphery in which the linear velocity becomes high is set higher, whereby a system is proposed for regenerating a disk of the CLV format even when the rotation velocity is constant or changed for a small amount (Japanese Patent Laid-Open No. 06-89506 gazette, Japanese Patent Laid-Open No. 06-12785, and the like). Furthermore, as described in Japanese Patent Laid-Open No. 08-212691 gazette, a method is proposed in which the disk rotation velocity is constant when recording data, and the clock frequency of a recording signal is changed according to the linear velocity, thereby enabling a high-speed recording is proposed. In this recording method, since the linear velocity changes depending on a disk radius, there occurs a necessity of optimizing a recording condition in conformity to the linear velocity. In Japanese Patent Laid-Open No. 08-212691 gazette, Japanese Patent Laid-Open No. 10-106025 gazette and the like, the recording condition is optimized by changing a characteristic of a recording medium between the inner and outer peripheries. However, there was a problem that this recording medium is an optical disk dedicated medium for controlling the rotation velocity at the constant disk rotation velocity (or substantially constant), thus the use is restricted. As means for solving these problems, in Japanese Patent Laid-Open No. 10-106008 gazette, a method is proposed that the recording condition is not optimized by the recording medium, but a power or a pulse width when recording is changed in accordance with the linear velocity, thereby forming a record mark of an optimal shape.
Even in the ZCLV system in which the linear velocity is substantially constant, in order to attain high density, a system of changing the recording pulse width when forming the record mark is adopted. As described on page 70 in the material of The Sixth Phase Change Optical Information Storage in 1994, this is a system (adaptive type recording waveform control) of recording a recording pulse shape by increasing or decreasing at a length shorter than clock unit, depending on a preceding space and/or a succeeding space. In this system, it is possible to restrict an edge shift as low as possible due to thermal interference in a track direction generated from the record mark or space being smaller than an optical spot, which is very effective for a mark edge recording.
In the case where in a phase change disk ensured that a jitter is good (about 13% or less) as necessary and sufficient under the condition that the linear velocity is substantially constant, as shown in Japanese Patent Laid-Open No. 10-106008 gazette, the recording is made by setting the disk rotation velocity constant and changing the clock frequency of the recording signal in accordance with the linear velocity, we examined the method for recording by changing the power or the pulse width of the recording pulse in conformity to the linear velocity. As the result, by this method, the jitter was reduced to a certain recording density as necessary and sufficient, namely at about 13% or less. However, for example, in the optical disk of the high recording density like the DVD-RAM disk of 4.7 GB or more, it was understood that it is impossible to obtain the sufficient and good jitter only by unequivocally changing the power and the pulse width of the recording pulse with respect to the linear velocity. As described above, in the DVD-RAM of 4.7 GB, the adaptive control recording system is introduced in a state that the linear velocity is substantially constant. If the linear velocity is changed at the same recording density, the thermal interference condition in the track direction changes. Therefore, it was impossible to obtain the good jitter only by unequivocally controlling the pulse width or recording power.
The object of the present invention is to solve the drawbacks in the above-described prior art and to provide an optical disk unit that comprises a recording system for enabling an accurate mark edge recording in the case of an optical disk of high recording density and variable linear velocity.